1. Field of the Invention
The present invention relates generally to digital filters, and more specifically, to digital decimation filters used in high-frequency applications.
2. Background of the Invention
Digital filters have replaced traditional analog filters in many applications. Predictability/repeatability and high-order slope implementations have made digital filters preferable to analog filters for both low-frequency and high-frequency applications. In particular, high-frequency digital filters are increasingly used in the intermediate frequency (IF) stages of radio communications equipment, wireless networking equipment and other radio-frequency (RF) applications. Various topologies have been used and proposed for use within IF stages of receivers, and decimating filters are particularly applicable to IF receivers as the required clocking rate of each successive stage may be reduced, resulting in a more economical solution.
In particular, a topology known as a Hogenauer Filter (as described by Eugene B. Hogenauer in “An Economical Class of Digital Filters for Decimation and Interpolation” published in the Institute of Electrical and Electronics Engineers (IEEE) Transactions on Acoustics Speech and Signal Processing (ASSP) April 1981, is desirable for use in receiver digital IF stages. The Hogenauer filter provides advantages in via a simple structure that lends itself to high-speed implementation, the decimation ratio is inherently scaled to the filter bandwidth and that the resources required are independent of the decimation ratio.
However, Hogenauer filters have several disadvantages. First, the DC gain of such a filter is kN where k is the decimation factor and N is the number of filter stages. So, as the decimation or length of the filter is increased, the DC gain must be compensated by lengthening the integrator. Second, the frequency domain transfer function approximates a sinc function: (sin πf/πf)N, which is not a particularly steep cut-off response for a given order of filter. Therefore, many filter stages are typically used in cascade to attain a particular level of stopband attenuation and rejection slope. Further, in the Hogenauer filter, the zeros of the filter are located at integer multiples of the decimated output rate (frequency). Therefore, once the filter order is determined, the rejection slope and ultimate stopband attenuation is set.
Therefore, it would be desirable to provide an improved digital decimation filter having improved rejection characteristics without greatly increasing the complexity of the filter. It would further be desirable to provide a digital decimation filter having positionable zeros, whereby the steepness of the cut-off response may be optimized for particular applications. It would further be desirable to provide an improved digital decimation filter having a selectable decimation rate.